Phase-locked-loop (PLL) devices are control systems that generate signals having a fixed relationship to the phase of a reference signal. Typically, a phase-locked loop device generates a desired signal in response to both the frequency and the phase of the reference signal as well as a control signal. Often this includes raising or lowering the frequency of a voltage controlled oscillator (VCO) until a modified form (a fraction, for example) of the VCO signal is matched with the reference signal in both frequency and phase. Phase-locked loops are widely used in radio, telecommunications, computers, and other electronic applications.
PLL devices may include a lock detector which compares the frequency of the modified output signal (e.g., the output of the VCO divided by a divider value) to the frequency of the reference signal, to determine whether the PLL device is “locked” to the desired frequency. The lock detector may output a “lock OK” signal when the compared frequencies are close or within an acceptable tolerance. Generally, the lock detector may be designed with a wide enough tolerance to accept certain lags and inconsistencies that may occur with sigma-delta modulator-based dividers, for example. Consequently, isolated and sporadic errors of the PLL device (such as a divider error, for example) may not be detected by the lock detector.
Other error detection methods may be used that include measurement of the VCO frequency spectrum or analysis of the VCO control voltage. Measurement of the VCO frequency spectrum may only detect those errors that fall outside of an acceptable range for the PLL device or for the particular application. Further, errors could go undetected for a substantial time, resulting in incorrect operation of the electronic appliance. Analysis of the VCO control voltage can be problematic, and may not yield the desired results. For example, on one hand, the VCO control voltage may be an analog signal that is not easily processed without additional analog to digital processing. Additionally, the VCO control voltage may maintain a predictable profile even while errors are occurring within the PLL, such as with an offset current.